1. Field of the Invention
This invention relates to electrical inductive apparatus and, more particularly, to encapsulated electrical inductive apparatus and methods for making the same.
2. Description of the Prior Art
Electrical inductive apparatus, such as transformers, reactors or the like, generate considerable quantities of heat during their operation which must be adequately dissipated if the device is to operate reliably. Many different methods are used to remove this heat, including circulating air or coolant fluid around the electrical inductive apparatus. One method used extensively with small transformers is to encapsulate the transformer in a case with a solid potting compound. This potting compound has higher thermal conductivity properties than air or oil and, as such, conducts considerable quantities of heat away from the transformer to the walls of the enclosure where it is carried off into the surrounding air. The usual method of encapsulating electrical inductive apparatus makes use of liquid synthetic resins which can be cured at a high temperature to a solid form. Although these types of materials are easy to work with and form a dense, solid mass, they are expensive and are also susceptible to shrinkage during the cure operation. This shrinkage, typically from 2 to 10% of the original volume, opens up voids or cracks between the enclosure and the transformer, which act as thermal barriers and impede the dissipation of heat away from the electrical inductive apparatus.
To lessen the detrimental effects of resin shrinkage, various inert, filler materials have been added along with the liquid resin to form the encapsulating compound. These inert materials, typically sand, alumina, mica or the like, are added in large amounts to reduce the quantity of resin used, and thereby lessen shrinkage. An example of this type of encapsulating compound is disclosed in U.S. Pat. No. 2,941,905, in the name of C. F. Hofmann and assigned to the assignee of the present application, wherein an inert filler material, such as sand, is poured into an enclosure which contains a transformer. A sufficient quantity of liquid resin is then added to completely impregnate the interstices between adjoining sand particles before the entire assembly is subjected to a high temperature for the length of time required to cure the resin. The resulting compound contains between 70 to 90% by weight of the inert filler material which gives it excellent resistance to crack formation and improved thermal conductivity.
Continued efforts have been made to increase the percentage of inert material in the potting compound to thereby improve its heat dissipation properties and further reduce shrinkage while at the same time providing a complete fill of all the interstices between adjoining particles. One such compound, as disclosed in U.S. Pat. No. 3,161,843 to Hodges, Antalis and Wood, uses resin-coated sand to form the encapsulating compound. In this type of encapsulating compound, a thermosetting resin compound is applied to each sand particle and partially cured such that each particle is covered with a thin film of dry resin. This resin coating, known as a "B" stage resin, is dry at ordinary room temperatures, but enters a fluid state when subjected to an elevated temperature and flows between adjoining sand particles to form cohesive bonds at the points of contact between adjoining particles as it hardens or cures. Since the resin coating constitutes only 5% of the weight of the coated sand particles, interstices result between the sand particles which are then filled with another insulating material. Although utilizing less resin than prior encapsulating compounds, this formulation uses additional material to completely fill the interstices between adjoining sand particles. This not only increases the cost of the encapsulating compound but adds additional manufacturing operations and could result in uneven heat dissipation if the insulating material does not completely fill all of the interstices.
A different method of improving the thermal conductivity of a potting compound involves the addition of a second filler material, such as gravel, in place of a portion of the sand. After the sand and gravel particles are poured into the case, according to this method, a sufficient quantity of liquid resin is then added to completely fill all of the interstices and wet all of the particle surfaces; which, when cured, binds the particles together into a solid mass. In order to obtain an even distribution of the sand and gravel particles throughout the encapsulating compound, it is necessary to mix the sand and gravel together before they are added to the enclosure since it has been heretofore impossible to get the sand particles to disperse evenly throughout the larger gravel particles when both materials are added separately to the enclosure. However, subsequent handling and pouring of the premixed sand and gravel mixture causes the larger gravel particles to separate from the sand particles, thereby creating concentrations of sand and gravel in the encapsulating compound, which causes voids and uneven heat dissipation. Furthermore, to insure complete wetting of the gravel and sand particles with the liquid resin, the components must be added to the enclosure in layers, that is, a layer of premixed sand and gravel followed by a small amount of liquid resin and then another layer of sand and gravel and continuing until the entire electrical inductive apparatus is covered with the encapsulating compound. Although less expensive than other types of encapsulating compounds, the separation of the gravel from the sand experienced with this method causes hot spots in the electrical inductive apparatus due to the uneven distribution of sand and gravel particles throughout the compound. Furthermore, the present method of manufacturing such an encapsulating compound is complex and time consuming.
Thus, it is still desirable, and it is an object of this invention, to provide an encapsulated electrical inductive apparatus wherein the encapsulating compound exhibits a high degree of thermal conductivity along with reduced shrinkage and which provides these characteristics at less cost than prior art encapsulating compounds.
It is also desirable and it is another object of this invention, to provide a new method for encapsulating electrical inductive apparatus with a compound containing certain large, inorganic particles and certain finely divided, resin-coated, inorganic particles; which method provides an even dispersion of both types of particles throughout the encapsulating compound and affords a simplified manufacturing operation.